📅 OpEx Dynamics

OpEx — monthly options expiration — falls on the third Friday of every month. A large fraction of open interest rolls off that day, which mechanically reshapes the gamma board. Weekly expirations do the same thing on a smaller scale every Friday. The two effects worth knowing are pinning into OpEx and the post-OpEx drift that follows once the expiring gamma is gone.

$Projection Heatmap \u2014 expected price path over the GEX grid with call-gamma pins and put-gamma walls$

Pinning into OpEx#

When a strike has dominant open interest and dealers are long gamma there, their hedging is mean-reverting. As price drifts above the strike they sell; as it drifts below they buy. The result is often a tight range around the strike into Friday's close.

GammaBaba GEX heatmap for SPY — gamma exposure by strike and expiration with call/put walls and the King Strike

The post-OpEx drift#

After Friday's close, the bulk of expiring open interest is gone. The board re-weights toward later expirations, and because dealers often held net long gamma in the expiring tenor, aggregate gamma usually drops. Less mean-reverting hedging = bigger realized moves the following week — this is the widely observed post-OpEx weakness effect.

How to read it on GammaBaba#

Open the GEX Multi-Panel during OpEx week to compare the expiring tenor against the next two.
Use the GEX Multi-Panel over Friday → Monday to quantify how much aggregate gamma rolled off.
On Monday, check whether the new King Strike is meaningfully far from spot — that extension often sets the week's tone.